1. Field of the Invention
The present invention relates to a receiving apparatus and method for receiving a high frequency signal suitable for use in various kinds of wireless appliances which perform the digital signal processing in, for example, base band at a high degree, and more particularly, to a technique for controlling the gain of a received signal.
2. Description of the Related Art
In a receiving apparatus for receiving and processing a ratio signal and so on, a gain-variable amplifier for amplifying an input signal in a gain variable fashion is provided in a receiving circuit system in order to keep the input level of a receiving signal at a constant level, and an automatic gain control circuit (AGC circuit) for controlling the gain of the amplifier corresponding to the level of input signal is incorporated thereinto.
Particularly, in case of a communication appliance for receiving digital data, it is very important to control a receiving signal level appropriately, because if the input level of an analog/digital converter for converting the received signal into digital data is not appropriately controlled, the analog/digital converter can not perform the digital conversion appropriately.
Meanwhile, if an amplifier for amplifying a high frequency input signal (RF signal) is as gain-variable, for example, is composed of a most general bipolar transistor, it is difficult to perform the gain-controlling successively, while maintaining the linearity of amplification characteristic of the amplifier itself. Further, since the AGC circuit is such a circuit as to apply a feedback in an analog manner, there exists a problem that it takes time for the system to converge to a stable state for a sudden variation of the input level.
In another manner using a gain-variable amplifier for controlling the gain successively, a circuit may be incorporated which performs the process of cutting a bias current of an RF amplifier using a non-linear element (so called shunt process) when a strong input having a level higher than a certain signal level is received, to thereby prevent an amplification from being performed in the amplifier. The case of such a circuit, the linearity of signal is excellent in any case because the amplification in the amplifier element is not performed at a time of a strong input.
However, the case that an RF amplifier for performing such a shunt process is incorporated into a prior art receiving circuit, it is necessary to detect the input level at the front stage of the RF amplifier to be shunted in order to cope with such a condition that when the input level decreases, while the RF amplifier does not perform the amplification due to the detection of a strong input, and therefore, an amplifier for amplifying the input signal and a detecting circuit for detecting the amplified output thereof are necessary in parallel to the RF amplifier to be shunted, resulting in a problem that the control arrangement becomes very complicated. Moreover, there exists a problem that if the shunt process is performed in such an RF amplifier, the input level may change sharply and therefore the receiving system becomes unstable due to the level fluctuation just after such a shunt process.